1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a facsimile machine, a laser printer, etc., and more particularly, to a developer-supplying system of an image forming apparatus having a residual developer-withdrawing member to remove residual developer which is not transferred onto a photosensitive body, but remains on a developing roller to be removed therefrom by a developer-supplying roller and to be moved onto the developer-supplying roller after developing.
2. Description of the Related Art
Generally, a non-contact image forming apparatus using one component nonmagnetic developer, as shown in FIG. 1, includes a photosensitive body 1 in the form of a drum rotating at a predetermined speed and direction (arrow A), a charging unit 2 applying a high DC/AC voltage onto a surface of the photosensitive body 1 to electrify the photosensitive body 1 to a predetermined electric potential, a laser scanning unit (not shown) forming an electrostatic latent image having a relatively high electric potential as compared to the surface of the photosensitive body 1 by scanning a laser beam 3 onto the surface of the photosensitive body 1 to discharge some electric potential therefrom, and a developing unit 4 attaching a developer 8 to the electrostatic latent image formed on the scanned surface of the photosensitive body 1 to form a visible image. The conventional apparatus further includes a transfer unit 9 transferring the developer 8 in the form of the visible image formed on the surface of the photosensitive body 1 onto a sheet of paper 13, a fixing unit (not shown) fixing the developer 8 transferred onto the sheet of paper 13 by heat and pressure, and a paper-discharging unit (not shown) discharging the sheet of paper 13 to the outside.
The developing unit 4 is provided with a developer-storing part 17 storing the developer 8, and a developing roller 5 disposed to rotate in a direction (arrow B) opposite to the rotation direction of the photosensitive body 1 and spaced a predetermined gap d from the photosensitive body 1, and connected to a development bias power source 12 to apply a development bias voltage in which AC voltage and DC voltage are added together. The developing unit 4 further includes a developer-supplying roller 6 disposed to rotate in a same direction as the rotation direction of the developing roller 5 coming in contact with the developing roller 5, a developer layer regulating member 7 regulating a thickness of a developer layer formed on the developing roller 5, and an anti-leak member 15 disposed below the developing roller 5 to prevent the developer 8 from leaking out.
The operation of the conventional image forming apparatus constructed as above will be described.
First, while the photosensitive body 1, of which the surface is electrified in a predetermined, for example, negative (−) electric potential by the charging unit 2, is rotated in the direction of arrow A, i.e., clockwise, an electrostatic latent image having a relatively-positive (+) electric potential is formed on the surface of the photosensitive body 1 by emitting the laser beam 3 from the laser scanning unit onto the surface of the photosensitive body 1 according to an image signal.
On the other hand, the developer 8 having a predetermined, for example, negative (−) polarity is moved onto a surface of the developing roller 5 by the developer-supplying roller 6 rotating in the direction of arrow B, i.e., counterclockwise, and is then attached thereon by an electric potential difference between the developer-supplying roller 6 and the developing roller 5 to which the development bias voltage is applied by the development bias power source 12.
Subsequently, the developer 8 attached on the surface of the developing roller 5 is frictionally electrified, and at the same time, is made in a thin developer layer having a uniform thickness, by the developer layer regulating member 7 coming in linear pressure contact with the developing roller 5.
Next, as the developing roller 5 continues to rotate, the developer 8 is moved into a development region in which the photosensitive body 1 faces the developing roller 5, and is attached to the electrostatic latent image due to an electric force generated by an electric potential difference between the electrostatic latent image formed on the surface of the photosensitive body 1 and the development bias voltage applied on the developing roller 5, to thereby form a visible image.
As the photosensitive body 1 rotates, the developer 8 attached to the electrostatic latent image is transferred onto the sheet of paper 13 supplied between the photosensitive body 1 and a transfer roller of the transfer unit 9, by a high voltage having a polarity opposite to that of the developer 8, i.e., a positive (+) polarity which is applied to the transfer roller of the transfer unit 9 disposed below the photosensitive body 1.
Next, the photosensitive body 1 continuously rotates so that a cleaning blade 10 removes developer remnants 8′ which remain on the surface of the photosensitive body 1, and after the developer 8 transferred to the sheet of paper 13 is fixed by the fixing unit, the sheet of paper 13 is discharged to the outside by the discharging unit.
However, according to the conventional image forming apparatus, during developing, the developer 8 is not moved and attached to the electrostatic latent image from a surface in a non-developing region of the developing roller 5, but instead remains thereon. Thus, when the developer 8 remains on the surface in the non-developing region of the developing roller 5, if the developing operation is repeated, the residual developer 8 is over-electrified, so that when a developer layer is regulated by the developer layer-regulating member 7, uneven electrical potential values occur, thereby resulting in a problem that development efficiency deteriorates and thereby a density of the resultant image is lowered.
To prevent this problem, the conventional image forming apparatus is constructed so that the developer-supplying roller 6 withdraws the residual developer 8 from the developing roller 5 after developing. However, since the developer-supplying roller 6 is usually formed of a foam elastic material such as urethane foam, silicon foam, or sponge, when the developer-supplying roller 6 repeats the operation of withdrawing the residual developer 8 from the developing roller 5 for a long time, minute vacant spaces or holes in a surface of the developer-supplying roller 6 are filled with the residual developer 8, so that elasticity of the developer-supplying roller 6 deteriorates and hardness thereof is increased.
Thus, when the elasticity of the developer-supplying roller 6 deteriorates and the hardness thereof is increased, the developer-supplying roller 6 presents a problem that developer-supplying efficiency to supply the developer 8 onto the developing roller 5 and residual developer-withdrawal efficiency to withdraw the residual developer from the developing roller 5 are not only lowered, but also stress to the developer 8 is increased to deteriorate the developer 8.